Mechanics and mechanisms of fatigue in a WC-Ni hardmetal and a comparative

study with respect to WC-Co hardmetals

J.M. Tarragó1, 4, C.Ferrari1,§, B. Reig2, D. Coureaux1, ф, L. Schneider3, L. Llanes1, 4, †

1 CIEFMA – Universitat Politècnica de Catalunya, Barcelona 08028, SPAIN

2 Sandvik Hyperion - Sandvik Española S.A, Martorelles 08107, SPAIN

3 Sandvik Hyperion, Coventry CV4 0XG, UK

4 CRnE - Universitat Politècnica de Catalunya, Barcelona 08028, SPAIN

† Author to whom all correspondence should be submitted.

§ CASC-Imperial College London, South Kensington Campus, London SW7 2AZ, UK

Ф Universidad de Oriente, Facultad de Ingeniería Mecánica, Santiago de Cuba, Cuba

Phone: +34-934011083; Fax +34-934016706

E-mail: luis.miguel.llanes@upc.edu

Abstract

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides)

aiming for materials with similar or even improved properties at a lower price. Nickel is

one of the materials most commonly used as a binder alternative to cobalt in these

metal-ceramic composites. However, knowledge on mechanical properties and

particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In

this study, the fatigue mechanics and mechanisms of a fine grained WC-Ni grade is

assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are

determined, and the attained results are compared to corresponding fracture toughness

and flexural strength. An analysis of the results within a fatigue mechanics framework

permits to validate FCG threshold as the effective fracture toughness under cyclic

loading. Experimentally determined data are then used to analyze the fatigue

susceptibility of the studied material. It is found that the fatigue sensitivity of the WC-

Ni hardmetal investigated is close to that previously reported for Co-base cemented

carbides with alike binder mean free path. Additionally, fracture modes under stable and

unstable crack growth conditions are inspected. It is evidenced that stable crack growth

under cyclic loading within the nickel binder exhibit faceted, crystallographic features.

This microscopic failure mode is rationalized on the basis of the comparable sizes of the

cyclic plastic zone ahead of the crack tip and the characteristic microstructure length

scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder

mean free path.

Keywords: WC-Ni hardmetal, fatigue mechanics, fatigue crack growth, fatigue

strength, fatigue mechanisms, fatigue sensitivity.

1. Introduction

Since the emergence of the first WC-Co cemented carbides in 1923, cobalt has

been the dominating metal used as binder in these metal-ceramic composite materials,

also referred to as hardmetals [1]. This is due to the especially favorable chemical

bonding between tungsten carbide and cobalt that results in a very low interfacial

energy, nearly perfect wetting and a very good adhesion in the solid state [2]. However,

the toxicity and high price of cobalt metal together with the need for improving the

performance of cemented carbides under severe working conditions, such as corrosion

and high temperature, have promoted the search and usage of grades with alternative

binders [3–5]. Among them, nickel has received the most attention as an alternative

binder to cobalt because of its similarity in structure and properties, besides its good

corrosion resistance. Proof of that is the increasing number of research papers focussed

on Ni-base cemented carbides published in recent years (e.g. Refs. [6–12]). Both cobalt

and nickel exhibit good wettability with WC, and fully dense hardmetals without

anomalous porosity can be produced [3]. The principal difference between them is the

higher stacking fault energy of Ni that results in lower hardening rates [5]. Thus,

hardness and strength of WC-Co grades tend to be superior to those exhibited by WC-

Ni ones. However, an increase of the work hardening rates of the Ni binder may be

achieved by means of minor and moderate additions of other elements such as

chromium [3] or silicon [7], yielding as a result similar or even superior hardness and

fracture strength levels for Ni-base cemented carbides, as compared to those exhibited

by plain WC-Co grades. Furthermore, Cr additions result in a large increase of the

corrosion resistance of WC-Ni hardmetals [12].

On the other hand, a better understanding of service degradation phenomena in

hardmetals is required for industrial manufacturers, if material performance and lifetime

of tools and components are to be improved. Among them, premature fatigue failure is

an important one since cemented carbides are commonly used in applications involving

high cyclic stresses (e.g. Ref. [13]). Fracture and fatigue behavior of hardmetals has

been extensively rationalized within the Linear Elastic Fracture Mechanics (LEFM)

framework, since failure of these brittle materials is governed by unstable propagation

of preexisting flaws (e.g. Refs. [14-17]). Following these ideas, and taking into account

that subcritical crack growth is the controlling stage for fatigue failure in cemented

carbides [18], Torres and co-workers proposed the fatigue crack growth threshold as the

effective toughness under cyclic loading [19]. Experimental validation for such

approach was then presented for a series of WC-Co hardmetal grades [20]. Moreover,

such results pointed out a strong microstructural influence on the fatigue sensitivity of

hardmetals, depending on the compromising role played by the metallic binder as both

toughening and fatigue susceptible agent [21]. Schleinkofer et al. [18] reported that as a

result of the accumulation of plastic deformation and/or due to high stresses during

cyclic loading, cobalt binder martensitically transforms from the FCC structure to the

HCP one. This deformation micromechanism restricts significantly the ductility of the

metallic binder, recognized as the main toughening phase in cemented carbides [17,22-

24]. On the other hand, nickel binder accumulates deformation in the form of slip plus

twinning damage mechanisms [25-27], but without evidence of such transformation.

Thus, it is not clear whether above relationships, regarding either fatigue mechanics

perspective or microstructural influence on the basis of binder mean free path, may be

directly extrapolated to hardmetals other than the Co-base previously studied. To the

best knowledge of the authors, there is not any information about fatigue strength and

fatigue crack growth behavior of WC-Ni cemented carbides in the open literature. It is

then the aim of this investigation to study the fatigue mechanics and mechanisms of a

Ni-base hardmetal grade.

2. Experimental aspects

The investigated material is a fine-grained WC-Ni hardmetal with a minor

addition of chromium. The key microstructural parameters: binder content (%wt), mean

grain size (dWC), carbide contiguity (CWC), and binder mean free path (λbinder) of the

studied material are listed in Table I. Mean grain size and carbide contiguity were

measured following the linear intercept method using Field Emission Scanning Electron

(FE-SEM) micrographs at a magnification of X4000, whereas binder mean free path

was estimated from empirical relationships given in the literature on the basis of

empirical relationships given by Roebuck and Almond [28], but extending them to

include carbide size influence [29,30].

Mechanical characterization includes hardness (HV30), flexural strength (σr),

fracture toughness (KIc), fatigue crack growth (FCG) parameters and fatigue limit (σf).

Hardness was measured using 294N Vickers diamond pyramidal indentations. In all the

others cases, testing was conducted using a four-point bending fully articulated test jig

with inner and outer spans of 20 and 40 mm respectively. For the determination of

flexural strength and fatigue limit, 45x4x3 mm beams were used. The surface which

was later subjected to the maximum tensile loads was polished to mirror-like finish and

the edges were chamfered to reduce their effect as stress raisers. For both experimental

sets, 15 samples were tested. Flexural strength tests were conducted on an Instron 8511

servohydraulic machine at room temperature and the results were analyzed using

Weibull statistics. Experimental fatigue limit (“infinite fatigue life” defined at

2 x 106 cycles) was assessed following the stair-case method. Tests were performed

using a resonant testing machine, at load frequencies around 150 Hz and under a load

ratio (R) of 0.1. After failure, a detailed fractographic inspection was conducted by FE-

SEM on tested specimens in order to identify the nature, size and geometry of the

critical flaws. Fracture toughness and FCG parameters were determined using 45x10x5

mm single edge pre-cracked notch beam (SEPNB) specimens with a notch length-to-

specimen width ratio of 0.3. Compressive cyclic loads were induced in notched beams

to nucleate a sharp crack [31,32] and details may be found elsewhere [33]. The sides of

SEPNB specimens were polished to follow stable crack growth by a direct-

measurement method using a high-resolution confocal microscope. Fracture toughness

was determined by testing SEPNB specimens to failure at stress-intensity factor load

rates of about 2 MPa√m/s. FCG behavior was assessed for two different R values, 0.1

and 0.5. Fracture surfaces of the SEPNB specimens corresponding to stable and

unstable crack growth were also examined by FE-SEM to discern, analyze, and compare

damage mechanisms under different load conditions.

3. Results and discussion

3.1. Hardness, flexural strength and fracture toughness

Basic mechanical properties for the studied cemented carbide are listed in Table

II. WC-Ni hardmetal exhibits hardness and flexural strength values close to those found

in Co-base grades with a similar mean free path [19,34,35]. Such response is different

from trends indicated by other authors [36,37], and should be ascribed to the chromium

dissolved within the binder. It has been stated that minor and moderate additions of

chromium raise the hardness and load-deflection response of WC-Ni up to levels

exhibited by WC-Co grades [3] via solid solution in nickel. Moreover, the flexural

strength dispersion evidenced is rather small; and accordingly, the corresponding

Weibull analysis yields a relatively high value, indicative of similarly high reliability

from a structural viewpoint. Fractographic examination reveals critical defects for the

studied material (e.g. Figure 1) with an equivalent diameter (2acr) of about 10-25 μm. It

is in agreement with values estimated from a direct implementation of the LEFM

equation KIc = Yσr(acr)1/2 relating strength (σr), toughness (KIc) and critical flaw size

(acr) (see Table II) by considering defects as either embedded or surface circular cracks.

In this equation, Y is a geometry factor that depends on the configuration of the flawed

sample and the manner in which loads are applied. This sustains the use of LEFM for

rationalizing the fracture behavior of the cemented carbide studied here.

3.2. FCG kinetics

FCG rates are plotted against the range and the maximum applied stress intensity

factor, ∆K (Figure 2a) and Kmax (Figure 2b) respectively, for the two load ratios

studied. As it has been previously reported for WC-Co hardmetals [15,19,21,38,39], the

WC-Ni grade under consideration exhibits: (i) a large-power dependence of FCG rates

on ∆K, as indicated by the high m values within Paris relationship – equation (1) below

– (Table III), and (ii) subcritical crack growth at ∆K values much lower than fracture

toughness. Also, a very pronounced load ratio effect is observed in the dependence of

FCG on ∆K. However, as observed for other brittle-like materials, R effects are largely

reduced when plotting FCG against Kmax. This is an indication of predominance of static

over cyclic failure modes [21,37].

(1)

To assess the relative dominance of Kmax and ∆K as the controlling fatigue

mechanics parameters under fatigue, Kmax was expressed as ∆K/(1-R). It allows plotting

a modified Paris relationship with the form given in expression (2), where C’, p and q

are constants.

(2)

Factoring out (1-R)q as a constant, FCG data collapse onto a nearly single curve

for an optimal q value (Figure 3). Then C’ and p parameters can be deduced from the

least squares regression knowing that the slope of the curve is the addition of p and q.

Values for the FCG threshold (measured for FCG rates of 10-9 m/cycle), as well as for p

and q constants of the modified Paris relationship are listed in Table III. The larger

value of p in the modified Paris relationship indicates that Kmax governs fatigue crack

growth over ∆K, pointing out once again the above referred predominance of static over

cyclic failure modes.

3.3. Fatigue mechanics: FCG threshold – fatigue limit correlation

One of the main purposes of this investigation is to extend the FCG – fatigue life

relationship proposed and validated by Torres and co-workers for WC-Co cemented

carbides [19,20] to Ni based hardmetals to optimize a proper design and material

selection in fatigue-limited applications involving hardmetals. Although this might be

attempted following a damage tolerance methodology, it does not seem to be an

amenable route because the enormous prediction uncertainties associated with marked

power-law dependences of FCG rates on ∆K (or Kmax) as those shown in Figure 2.

Instead, a classical approach on the basis of fatigue limit, within an infinite life

framework, and FCG threshold (Kth) is implemented by defining the latter as the

effective toughness under fatigue for a given critical flaw size. Thus, fatigue limit is

deduced from the stress intensity factor threshold of a small non-propagating crack

emanating from a defect of critical size, 2acr, according to a relationship of type (3):

(3)

Hence, the fundamental LEFM correlation among strength, stress intensity factor

and defect size applies also for natural defects too in cemented carbides. This assertion

may be done considering that: (i) size of the critical natural flaws are larger than the

microstructural unit; (ii) plasticity is confined to process zone ahead the crack tip; and

(iii) process zone governing fracture (multiligament zone behind the crack tip) extends

over a relatively short distance (about five ligaments) [19,40]. Thus, fatigue limit values

can be estimated from the relation given by the expression (4) under the assumption that

flaws controlling strength have the same size, geometry and distribution under

monotonic and cyclic loading.

(4)

Attempting to validate the estimated fatigue limit, an experimental study was

conducted using 15 samples and following an up-and-down load (stair-case) fatigue test

(Figure 4). Predicted and experimentally determined fatigue limits are listed in Table

IV. FCG threshold – fatigue limit correlation is validated by the excellent agreement

attained between them. Furthermore, the fractographic examination conducted on failed

specimens reveals that size, geometry and nature of the critical defects are similar under

both monotonic and cyclic loading conditions. This supports prediction of fatigue limits

from the corresponding fatigue sensitivity, parameter here defined as [1 – (Kth/KIc)] and

ranging thus from 0 to 1. Within this context, fatigue sensitivity represents an index for

describing the susceptibility to mechanical degradation of a material when subjected to

cyclic loads.

3.4. Fatigue sensitivity

Llanes et al. [21] investigated the fatigue sensitivity - [1 – (Kth/KIc)] - and the

modified Paris law exponent ratio (q/p) of a series of WC-Co hardmetals as a function

of their binder mean free path and applied load ratio. The corresponding results are

plotted in Figure 5, together with the fatigue sensitivity and p/q ratio exhibited by the

studied WC-Ni cemented carbide. Results show that the fatigue sensitivity of the

studied Ni-base hardmetal is similar to that expected for a WC-Co grade with alike

binder mean free path. Furthermore, it is also evidenced that p/q ratio of the studied

WC-Ni grade fits the trend described by Llanes et al. for WC-Co cemented carbides

[21]. Thus, it appears that Ni and Co binders exhibit a similar cyclic degradation of

operative toughening mechanisms in corresponding hardmetals, although the nature of

their plastic deformation mechanisms is different. In this regard, it should be recalled

that plastic deformation mechanisms in the Ni binder include slip and twinning [25-27],

also discerned in the Co-base binders, but not stress-induced phase transformation, as it

is the case in WC-Co grades. Care should be taken on above statements as they are

based on the results obtained for a single WC-Ni grade. Within this context, further

research on additional WC-Ni hardmetals with different microstructural characteristics

is recalled for sustaining these ideas.

3.5. Fatigue mechanisms: crack – microstructure interaction

After failure, the fracture surfaces of tested specimens were examined using FE-

SEM. Clear differences are evidenced when comparing fractographic aspects

corresponding to stable (Figure 6) and unstable crack growth (Figure 7). While in the

former “step-like” fatigue damage features are discerned within the binder, in the latter

the metallic binder exhibit well-defined dimples, suggesting a pure ductile fracture

mechanism. Fracture under cyclic loading in the nickel binder follows a faceted,

crystallographic fracture mode, as can be appreciated by the sharp angular facets

localized within broken binder regions. This faceted, crystallographic fracture mode has

been previously reported in WC-Co hardmetals subjected to cyclic loads [21,39,41-43],

and has been ascribed to fatigue-induced phase transformation within the Co-base

binder. Although such hypothesis could be supported by the observation of similar

cleavage-like features in fracture surface of bulk cobalt-base alloys within the high

cycle fatigue regime, it is not conclusive as morphologies of FCC (deformation) twins

and HCP (phase transformation) lamellas exhibit similar morphologies, and phase

transformation seems to be enhanced with applied maximum stress levels [44]. Within

this context, very interesting is the fact that crystallographic stable crack growth paths

have also been reported in Ni-base alloys (e.g. Refs. [45-47]) in the near-threshold FCG

regime. In this case, and similar to the WC-Ni hardmetal grade here investigated, phase

transformation mechanisms cannot be invoked at the binder phase; pointing out the

step-like crack morphology to be rather a microstructure size scale effect. In this regard,

it is well-known that the transition from the near-threshold regime to the intermediate

stage: (i) is accompanied by a noticeable change from a microstructure-sensitive to a

microstructure-insensitive fracture behavior; and (ii) occurs when the size of the cyclic

plastic zone (rc) becomes comparable to the characteristic microstructural dimension of

the material under consideration [48]. When plane stress conditions are satisfied, the

size of the cyclic plastic region can be approximated as

(5)

where ∆KI is the applied stress intensity factor range and σy is the yield strength of the

material. In cemented carbides, the high effective yield stress exhibited by the

constrained binder (between 2 and 4 GPa [17]), together with the relatively low ∆KI

values at which stable crack growth takes place (between 4 and 10 MPam1/2, Figure 2),

yields a submicrometric plastic region ahead the crack tip, whose size is then

comparable to the binder mean free path (Figure 8). Under these conditions,

microscopic failure modes characterized by localized shear and zig-zag crack paths may

be expected, as it is evidenced in this investigation too. Furthermore, as similar

microstructure-plasticity scenario applies to WC-Co cemented carbides, the findings of

this study raises the question on the speculated critical role played by the FCC to HCP

phase transformation for rationalizing a relatively higher fatigue sensitivity of plain

WC-Co hardmetals as compared to grades constituted by other alternative binders (e.g.

Ref. [49]). Further research in this interesting issue is clearly required.

Conclusions

The fracture and fatigue behavior of a fine grained WC-Ni hardmetal has been

investigated and compared to that of Co-base cemented carbides with similar

microstructural parameters. The following conclusions may be drawn:

1. The studied WC-Ni (with minor chrome addition) hardmetal exhibits similar

hardness, transverse rupture strength and fracture toughness to those observed

for a Co-base grade with alike binder mean free path. Within this context, the

use of LEFM to rationalize the fracture behavior of Ni-base cemented carbides is

also validated.

2. As previously observed for plain WC-Co grades, the WC-Ni hardmetal studied

exhibits a large-power dependence of FCG rates on both ∆K and Kmax, as well as

subcritical crack growth at Kmax values lower than KIc. Moreover, values of

fatigue sensitivity and FCG kinetics parameters determined for the WC-Ni grade

are in satisfactory agreement with those estimated from microstructure – FCG

behavior trends proposed from fatigue data gathered from WC-Co cemented

carbides.

3. A fatigue mechanics analysis allows to estimate the fatigue limit of the WC-Ni

hardmetal investigated on the basis that Kth is the effective toughness under

cyclic loads.

4. Stable FCG for the hardmetal studied is characterized by faceted,

crystallographic features within the binder, different from the ductile dimples

evidenced in the region of unstable propagation. Such fatigue failure mode is

postulated to be a direct consequence of the comparable size length scales of

microstructure and cyclic plastic zone in front of the crack tip. The facts that

both fractographic scenario during stable FCG and fatigue sensitivity (for a

given binder mean free path) are similar for Ni-base and Co-base hardmetals

raises then the question on the speculated role played by the FCC to HCP phase

transformation as a critical fatigue micromechanism in WC-Co cemented

carbides.

Acknowledgements

This work was financially supported by CDTI (National Board for Technological and

Industrial Development) within the CENIT Spanish program (Forma0) as well as by the

Ministerio de Economía y Competitividad (Grant MAT2012-34602). The authors,

Sandvik Hyperion (SH) and Universitat Politècnica de Catalunya (UPC), acknowledge

the work and support of all the members of the Forma0 consortium, led by SEAT.

Additionally, J.M. Tarragó and D. Coureaux acknowledge the scholarships received

from UPC/SHM and the Agencia Española de Cooperación Internacional (MAEC-

AECID), respectively.

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List of Tables

Table 1. Microstructural parameters for the WC-Ni cemented carbide studied.

Table 2. Hardness, strength and fracture mechanics parameters for the WC-Ni

hardmetal investigated.

Table 3. FCG threshold and Paris law parameters for the WC-Ni hardmetal studied.

Table 4. Predicted and experimentally determined fatigue limit values in terms of

maximum applied stress and fatigue sensitivity.

List of figures

Figure 1. Example of critical flaw (binderless carbide agglomerate) that originates

fracture in the WC-Ni hardmetal investigated.

Figure 2. da/dN behavior vs. (a) ∆K, and (b) Kmax, for each load ratio studied.

Figure 3. Normalized FCG rate as a function of Kmax.

Figure 4. Up-and-down fatigue test used to determine mean fatigue limit for the WC-Ni

cemented carbide studied.

Figure 5. Fatigue sensitivity (left and dashed lines) and modified Paris law exponents

ratio (q/p) (right and solid line) as a function of binder mean free path for the WC-Ni

hardmetal studied (orange) as well as for the WC-Co grades investigated by Llanes et

al. [21].

Figure 6. Scanning electron micrographs corresponding to stable crack growth (R=0.1)

for the WC-Ni hardmetal investigated. Fatigue facets are neatly discerned in the

metallic constitutive phase.

Figure 7. Scanning electron micrograph corresponding to unstable crack growth for the

WC-Ni hardmetal studied. Ductile dimples are evidenced within the binder.

Figure 8. A schematic representing cyclic plastic zone - microstructure length scales

existing at the crack tip during stable FCG in cemented carbides.

Figure 1. Example of critical flaw (binderless carbide agglomerate) that originates

fracture in the WC-Ni hardmetal investigated.

Figure 2. da/dN behavior vs. (a) ∆K, and (b) Kmax, for each load ratio studied.

(a) (b)

Figure 3. Normalized FCG rate as a function of Kmax.

Figure 4. Up-and-down fatigue test used to determine mean fatigue limit for the WC-Ni

cemented carbide studied.

Figure 5. Fatigue sensitivity (left and dashed lines) and modified Paris law exponents

ratio (q/p) (right and solid line) as a function of binder mean free path for the WC-Ni

hardmetal studied (orange) as well as for the WC-Co grades investigated by Llanes et

al. [21].

Figure 6. Scanning electron micrographs corresponding to stable crack growth (R=0.1) for the WC-Ni hardmetal investigated. Fatigue facets are

neatly discerned in the metallic constitutive phase.

Figure 7. Scanning electron micrograph corresponding to unstable crack growth for the WC-Ni hardmetal studied. Ductile dimples are evidenced

within the binder.

Figure 8. A schematic representing cyclic plastic zone - microstructure length scales

existing at the crack tip during stable FCG in cemented carbides.

Plastic zone

Crack tip

Binder

WC carbides